As a semiconductor device used in an inverter and other power converter, there are an IGBT (insulated gate bipolar transistor), a diode, and other devices. The diode is typically used for reflux and connected to an IGBT in parallel thereto but in the opposite orientation. The diode is therefore called an FWD (free wheeling diode).
To improve characteristics of an inverter and other power converters, it is important to improve characteristics of the FWD concurrently with characteristics of the IGBT. The characteristics to be improved include an ON-state voltage (voltage drop in conducting state), recovery time (annihilation time of recovery current at the time of reverse recovery), a safe operation region at the time of recovery (operation region where device is not broken even when voltage is applied in a state in which reverse recovery current flows), current/voltage oscillation at the time of recovery, and other parameters. Among them, it is important to shorten the recovery time. In the following description, the reverse recovery is called recovery.